Sign system adaptor



FIG. 1 is a perspective view of a sign system adaptor showing my new design;

FIG. 2 is a left side view of FIG. 1;

FIG. 3 is a front view of FIG. 1;

FIG. 4 is a top plan view of FIG. 1; and,

FIG. 5 is a bottom plan view of FIG. 1. 

The ornamental design for a sign system adaptor, as shown and described. 